In recent years, an electrically-driven vehicle equipped with a power source device including a plurality of battery cells as a power source for motor of a hybrid car or an electric vehicle has been popular. Furthermore, a power source device used for a storage system combined with a generator such as a solar cell and storing electric power generated by the generator has been also known. In these power source devices, as the battery cell, for example, chargeable and dischargeable secondary batteries such as a lithium ion battery and a nickel hydrogen battery are used.
Examples of the battery cell include a prismatic cell including an outer casing made of a metal outer can, a pouch cell including an outer casing made of a thin laminated film and having low rigidity, and the like. When a battery cell is in an abnormal state, an internal pressure of the battery cell increases and gas is generated in the outer casing.
The outer casing of the prismatic battery includes a bottomed metal can with one surface opened, and a sealing body for closing the opened surface of the metal can. The sealing body is provided with a safety valve. When pressure in the outer can increases, the safety valve is opened and gas can be discharged. On the other hand, an outer casing of a pouch cell is made of one laminated film. Specifically, a power-generating element such as an electrode plate is enclosed by a laminated film, and an overlapped portion of the laminated film is thermally welded so as to seal four sides. Thus, an outer casing is formed. In the battery cell having such a configuration, when an internal pressure of the outer casing of the battery cell increases, the thermally welded portion of the laminated film is peeled off or broken, and gas generated in the outer casing is discharged. Therefore, in the pouch cell, it is difficult to specify a position in which the gas is discharged.
In view of such circumstances, a pouch cell having a structure capable of specifying a gas discharged position in the thermally welded portion has been proposed. For example, in a pouch cell disclosed in the following Patent Literature, when a laminated film is thermally welded, a section whose adhesion strength of thermal welding is intentionally reduced is provided by, for example, a method of changing area thermally welded. Thus, a gas discharge portion is formed in a part of the thermally welded portion of the laminated film. In this configuration, when an internal pressure in the outer casing increases, the thermally welded portion is peeled off or broken from the section whose adhesion strength is low in the thermally welded portion. Accordingly, it becomes easy to specify a gas discharge position.